The present invention is in the field of optical frequency conversion using birefringent nonlinear crystals, and in particular, relates to using an uncoated, specially cut lithium triborate (LBO) crystal.
Phase matching in birefringent nonlinear crystals used for optical frequency conversion generally requires a nonlinear interaction between three linearly polarized optical beams propagating in the same direction, but with one of the beams polarized orthogonally to the other two. Usually anti-reflection (AR) coatings are applied to the entrance and exit faces of the crystal, though it can be difficult or impossible to obtain ideal AR coatings for three wavelengths. These optical coatings on nonlinear crystals, however, can be an impediment to using them as frequency-conversion devices at high average power. For example, absorption by coatings on the biaxial crystal lithium triborate (LBO) causes local heating of both the coating and the crystal. Because LBO is extremely transparent at near-infrared and visible wavelengths, coating absorption can be larger than absorption in the bulk crystal. This heating, because of the large and anisotropic thermal expansion of LBO, may cause coatings to buckle or loosen. (J. D. Beasley, Thermal conductivities of some novel nonlinear optical materials, Appl. Opt. 33, 1000-1003 (1994)).
It is an object of the present invention to avoid this heating problem by enabling uncoated birefringent crystals to be used for frequency conversion applications. It is a further object to obtain a sum-frequency-generated optical beam exiting with a polarization in the plane of incidence using an input of two linearly polarized optical beams.
The present invention uses three Brewster-cut uncoated planar surfaces on a specially cut birefringent crystal to obtain a third frequency by sum-frequency generation from a two-frequency input. Glass prisms, Brewster-cut on one end, normally cut on the opposite end, and appropriately coated, are placed adjacent to the birefringent crystal""s Brewster-cut faces so that the transverse profile of the beams is unaffected by refraction at the Brewster interfaces. The optical frequency conversion of high-powered input beams can be generated without danger of damaging the coatings normally used on birefringent crystals.